Capacitor sensors and optical sensors may be employed as an alternative to mechanical sensors for higher sensitivity. A conventional capacitor sensor includes a permanently charged membrane which forms a capacitor with a metal plate. This type of sensor is vulnerable to humidity, moisture, liquid leakage, dirt or other environmental factors which can cause a fault by shorting out and discharging the two sides of the membrane. In addition, to permanently charge the membrane, a voltage of around 10,000 V needs to be applied to the membrane, making such a sensor costly and difficult to manufacture and unsuitable for many applications.